Unlicensed Mobile Access (UMA) technology provides access to GSM and GPRS mobile services over unlicensed spectrum technologies, including Bluetooth and IEEE 802.11 (WLAN—Wireless Local Area Network). UMA technology allows service providers to enable subscribers to roam and handover between cellular networks and public and private unlicensed wireless networks using dual-mode mobile handsets.
In the following, the basic architecture of UMA is described by referring to FIG. 1. As illustrated, a terminal, e.g. a mobile station MS 1, obtains access to a core mobile network 3 via an Unlicensed Mobile Access Network (UMAN) 2. In detail, the mobile station 1 is connected to an Access Point (AP) 21, for example via Bluetooth or Wireless Local Area Network (WLAN, IEEE 802.11), as described above. A connection between the AP 21 and an UMA network controller (UNC) 23 is provided via a broadband Internet Protocol (IP) network 22. It is noted that the specific UMA features basically apply only for the MS 1 and the UNC 23 so that for the AP 21 any generic AP may be used.
The UNC provides functions which are basically equivalent to that of a Radio Access Network (RAN) Base Station Controller (BSC). The UNC interfaces into the mobile core network via existing A/Gb interfaces. The same mobile identity and cell identities (Cell ID) are used in both cellular RAN and in UMAN networks. A Security Gateway (SGW) 231 may be integrated in the UNC. The SGW terminates secure remote access tunnels from the MS, providing mutual authentication, encryption and data integrity for signaling, voice and data traffic.
Once a UMA capable dual mode terminal is registered to the UNC, from that point on all mobile voice and data traffic is routed to the terminal via the UMAN rather than the cellular radio access network (RAN). If the terminal has an active GSM voice call or GPRS data session when the terminal come within range (or out of range) of an unlicensed wireless network, that voice call or data session can automatically handover between access networks without interrupting the service.
As mentioned before, the UMAN network basically replaces the cellular RAN from the mobile core network point of view. A Mobile Services Switching Center (MSC) in the mobile core network sees a UNC as one of the BSCs and is able to use the same interface, i.e. the A interface, to communicate with UNCs and BSCs. Further, since the same GSM identity and GSM Cell ID are used in UMAN, the MSC is not able to distinguish, whether the current access technology is UMA or GSM, based on only the information exchanged over the A interface.
Handover between two access technologies are often called intersystem handover (ISHO). An example of a known intersystem handover is a handover between GSM and Universal Mobile Telecommunications System (UMTS). Handover between GSM and UMA can also be seen as intersystem handover.
When a mobile terminal performs a handover from GSM to UMA, it may happen that the UNC that is controlling the target/entered UMAN is connected to a different MSC (MSC-B) than the BSC that controlled the cellular RAN. In this case, the handover is called intersystem inter-MSC handover, because also the serving element (MSC) to which the BSC/UNC is connected in the mobile core network is changing. FIG. 2 illustrates an intersystem inter-MSC handover from GSM to UMA. Even though MSC-A does not directly interface a new BSC/UNC after the handover, MSC-A still remains involved in the call control. For example, mobility management and charging are still performed in MSC-A, and the active call may be handed over back to MSC-A.
Relevant interfaces between the network elements are also shown in FIG. 2. Both BSC and UNC connect an MSC using the A-interface of the GSM standard. Between two MSCs, an E-interface of the GSM standard is provided.
For charging and statistics it would be beneficial if MSC-A could know the current access technology in use. However, when an (intersystem) inter-MSC handover occurs from MSC-A to MSC-B (see “handover 1” illustrated in FIG. 3), MSC-A has no means to detect the access technology to which the handover was made. It may also happen that after a normal inter-MSC handover (from GSM based access to GSM based access, see “handover 2” illustrated in FIG. 3) from MSC-A to MSC-B, an MSC-B internal handover from GSM based access to UMA is made (see “handover 3” illustrated in FIG. 3). It is apparent that also quite a number of MSC-B internal handover may take place, wherein the above problem occurs every time, a not recognized change in the access technology happens.
Regarding the above mentioned known intersystem handover between GSM and Universal Mobile Telecommunications System (UMTS), it is to be noted that no solution to the above problem can be learned therefrom, here.
In (inter-BSC and inter-MSC) handover procedures, a mobile terminal measures and periodically reports to a base station (BS) the signal quality of the current and neighboring cells. The BS forwards the information to the BSC. If the BSC determines that the signal quality of the current cell is not good enough and a neighboring cell offering better signal quality is available, the BSC requests a handover. The BSC sends to the MSC a handover message including a list of neighboring cells to which the handover is possible to perform. The MSC manages the handover and chooses the destination cell taking into account also capacity and other requirements which the BSC was not able to consider. In an inter-BSC handover the MSC contacts a new BSC, and, in an inter-MSC handover also the MSC changes and the MSC (MSC-A) must contact another MSC (MSC-B) to assist in the handover.
If a mobile terminal supports multiple radio access technologies, for example GSM and UMTS, also an intersystem handover is possible, as described above. The difference to above described handover procedures is that the mobile terminal has to measure and report the signal quality of cells belonging to different radio access technologies. The decision to request a handover is made in the RAN. If a handover is to be requested, a handover target cell is identified differently in GSM and UMTS technologies. In UMTS, the handover target cell is identified using Service Area Identity (SAI), which includes Service Area Code (SAC). SAC uniquely identifies a service area within a location area in UMTS networks. In GSM, the handover target cell is identified using GSM Cell ID. Therefore, an MSC is always able to conclude, based on the identity of the handover target cell, whether the handover is to be performed to GSM or UMTS.
However, as described before, in UMA the same GSM Cell ID is used for identification in UMA. Hence, an MSC cannot determine from the handover target cell identity whether the handover is to be performed to GSM or UMA.
As indicated in FIG. 2, with respect to the GSM handover on the protocol level, the Mobile Application Part (MAP) is used for signaling between the mobile services switching centers (MSC) and registers in the mobile core network. For example, the E-interface uses MAP for signaling (see FIG. 3). MAP is used for location updates, call control of incoming calls to a mobile station, and transmission of short messages. MAP is not a protocol by itself, but a set of non-call-related signaling protocols.
The Base Station System Application Part (BSSAP) is a protocol that supports message communication between the mobile services switching center (MSC) and base station system (BSS) in A-interface. BSSAP consists of a base station management application part (BSSMAP) and a direct transfer application part (DTAP). In inter-MSC handover, BSSAP messages are encapsulated in MAP messages for transmitting handover related information between the BSSs of MSC-A and MSC-B.
BSSMAP is an application part which supports all procedures between a mobile services switching center (MSC) and base station system (BSS) that require interpretation and processing of information related to single calls and resource management. DTAP is a user part which transmits messages transparent as regards the base station system (BSS) between a mobile station (MS) and MSC. DTAP information is not interpreted by the BSS.
Referring again to FIG. 3, when a mobile station (MS) is handed over between two MSCs, the establishment of a connection between the MSCs requires interworking between A-interface and E-interface. The handover procedure is normally triggered by BSS-A by sending a HANDOVER_REQUIRED BSSMAP message on A-interface to MSC-A. The invocation of the basic inter-MSC handover procedure is performed and controlled by MSC-A. The sending of the MAP_Prepare-Handover request to MSC-B is triggered in MSC-A upon receipt of the HANDOVER_REQUIRED BSSMAP message from BSS-A. For compatibility reason, the cell identity of the cell where the call is to be handed over in MSC-B area, provided in the HANDOVER_REQUIRED BSSMAP message, is mapped into targetCellId MAP parameter and the HANDOVER_REQUEST BSSMAP message is encapsulated in Access Network Application Protocol Data Unit (AN-APDU) MAP parameter of the MAP_Prepare-Handover request. MSC-B informs BSS-B about the requested handover by sending HANDOVER_REQUEST BSSMAP message. MSC-B, after receiving an acknowledgment from BSS-B, responds to MSC-A with a MAP_Prepare-Handover response after which MSC-A orders the handover to be performed by sending a HANDOVER_COMMAND BSSMAP message to BSS-A.
If an MSC-B internal handover is performed later, MSC-B sends MAP_PROCESS_ACCESS_SIGNALLING request to MSC-A to pass handover related information received on A-interface of MSC-B. MAP_PROCESS_ACCESS_SIGNALLING request includes HANDOVER_PERFORMED BSSMAP message.